Glass strands are typically formed by attenuating glass filaments through bushing tips at the bottom of a heated bushing containing molten glass. The filaments pass across the face of an application surface where they are coated with a binder and/or size. The filaments are then gathered into a unified strand by means of a gathering shoe which may be, for example, a grooved bar or disc formed of a material such as graphite, traversed across the face of a rotating spiral and wound as a forming package around the mandrel or collet of a winder. The pulling forces from the collet provide the attenuative forces necessary to form glass filaments.
Often, the bushings are directly connected to a forehearth of a glass melting furnace, with the molten glass from the furnace then being directly attenuated into filaments. Such an operation does not require any pressurization of the molten glass beyond the natural head above the bushing of the glass, as the filaments are being formed from bushing tips at the bottom of the bushing of a size to allow the necessary flow rate of glass through the tips.
In a different type of glass strand formation, glass filaments are formed from much smaller orifices at much higher pressures. The pressure may be produced by maintaining a large column or head of glass above the bushing or by pumping the glass through the bushing. Typical of such glass fiber forming operations is the system disclosed in U.S. Pat. No. 3,573,014, which is assigned to the assignee of the present invention and incorporated herein by reference.
A typical pump for molten glass is a viscosity pump, such as that illustrated in U.S. Pat. No. 3,446,149, which is also assigned to the assignee of the present invention and incorporated herein by reference. This patent employs a rotating cylindrical element having a uniform inner surface and a stator within the cylindrical element having a plurality of channels in its surface with viscous fluid such as molten glass being pressurized by flowing between the two elements due to the viscosity drag of the molten glass. The pump requires that the cylindrical element, the stator, and any other part which contacts the molten glass be formed of a material such as platinum or a platinum-rhodium alloy to protect the elements from the corrosive nature of the molten glass. It is desirable, therefore, to produce a viscosity type pump which will reduce the amount of precious metal necessary in its construction and which employs relatively smooth surfaces between which the viscous fluid flows to reduce wear of the surfaces.
The use of pressurized glass in fiber forming has several advantages over attenuation from low pressure bushings. In a pressurized bushing the tips or orifices through which the filaments are attenuated are smaller in diameter for a given glass flow rate through the tips or orifices than with the low pressure bushings commonly employed. This smaller tip or orifice size results in a lower fiber tension during forming. In addition, the tips or orifices may be more closely spaced than previously possible. Further, more orifices may be located on a single bushing than were previously employed. All of these advantages lead to greater production capability with far smaller bushings.
The size of the bushing is highly important. Due to the corrosive nature of molten glass bushings must be formed of precious metals, such as platinum or platinum-rhodium alloys, which will withstand this environment. These materials are extremely expensive. Thus, the smaller the bushing area necessary for a given production level, the smaller the capital investment, and the greater the return on investment becomes. Therefore, employment of pressurized bushings may substantially increase the productivity and profitability of glass fiber formation.